7^ ^7 



/// 



l\ « 



'i^- 



(1.) To determine the homogeneousness of ilte material. 

Examine the form of the initial iDortion of the diagram between the 
starting point and the sudden change of direction which has been shown 
to indicate the elastic limit. Notice also its incKnation from the vertical 
and compare with it the inclination of the " elasticity line." 

A perfectly straight line beneath the elastic limit, perfectly j)arallel 
with the " elasticity line, " shows the material to be liomogeneous as to 
strain; i. e., to be free from internal strains, such as are produced by 
irregular and rapid coohng or by working too cold. Any variation from 
this line indicates the existence, and measures the amount, of strain. A 
line considerably curved as in No. 6, Plate I, exhibits the existence of 
such strain. 

Next, examine the form of the curve immediately after passing the 
elastic limit. A line rising from the elastic limit, regularly and smoothly, 
approximately parabolic in form and concave toward the base line, as in 
No. 22, indicates homogeneousness in structure, and the absence of such 
imperfections as are produced in wrought-iron by cinder, or in cast metals 
which have been worked from ingots, by porosity of the ingot. 

A line turning the corner sharply, when passing the elastic limit, and 
then running nearly or quite horizontal, as in other irons, and in the low 
steel of Plates II and III, or actually becoming convex toward the base 
line, as with some of the woods in Plate I, and then, after a time, re- 
suming upward movement by taking its proper parabolic path, indicates 
a decided want of this kind of homogeneity. The relative length of the 
depressed portion of the line, and the amount of dei3ression, measures 
the relative defectiveness of materials compared in this respect. 

Finally, compare the diagrams produced by several specimens of the 
same kind of material or from the same mass. Homogeneousness in 
general character and homogeneousness in composition are proven by the 
precise similarity of these diagrams, while a gTeater or less variation of 
the curves compared, indicates a greater or less difference in the speci- 
mens of which they are the autographs. 

Materials should usually exhibit great homogeneousness in all these 
three ways, to be perfectly reliable. Perfect homogeneousness is not to 
be expected in either respect. 

(2.) To determine the elastic resistance of ilie specimen. 

Measure the height of the curve at the elastic limit, using the scale of 
torsion, or for tension, which is given for each machine and for each 
■standard size of test-i^iece, as shown in the accompanying plates. 



(3.) To determine the resistance offered to any given amount of extension^ 
or that producing a given set. 

Measure the ordinate of the curve at the point whose abscissa, or 
distance from the origin, measures the assumed degree of set. 
(4.) To determine the idtimate resistance of the material. 

Measure in a similar manner the maximum ordinate of the curve. 

(5. ) To determine the resilience of the piece icitliin the elastic limit ; i. e. 
the work required to produce an evident and permanent set, approxi- 
mately proportional in amount to the degree of change of form of the 
specimen. This quantity measures the power of the material to resist 
blows, and its determination is evidently quite as important as that of 
resistance to static stress, which latter forms one of the factors of the 
former. 

Measure the area comprised between the ordinate of the curve at the 
elastic limit and the initial part of the curve ; this quantity is propor- 
tional to the required value. Or, multiply the elastic resistance of the 
material by the extension Avithin the elastic limit. As an approximate result, 
tv\-o-thirds this product is the quantity required in inch-pounds or foot- 
pounds, according as measures of extension are taken in inches or feet. 

(6.) To determine the resilience of the material loithin any assurned limit 
of extension; i. e., the magnitude of blow required to produce a 
given set. 

Measure the area of the curve up to the assumed limit, as for example, 
the area in Plate in, under No. 21, Z, 21, 21, Y, x, where the assumed 
set is the extension from Z to x. Two-thirds the product of the resist- 
ance measured by the altitude Kr, and the extension hx gives as before 
an api3roximate value for ordinary purposes. 

(7.) To determine the total resi'/^'e/ice or shock resisting power of the 
material. 

Measure the total area of the diagram. For ductile materials an 
apxDroximate value is obtained by taking two-thirds the product of the 
maximum tenacity by the maximum extension. For hard and very brittle 
materials one-haK the same product gives very accurately its values. 
For intermediate qualities the true value is more nearly two-thirds this 
product, also Swedish wi'ought-iron, white cast-iron, and hardened steel 
illustrate the first and the second classes ; ordinary tool steels are ex- 
amples of the third class, as is also iron like No. 22. 

(8.) To determine the effect of a load given in pounds per srpiare inch of 
stress. 



Find a point in the curve having an ordinate which measures the 
given stress. The abscissa of that point measures the extension under 
that load. In other words, a point being found in the curve, the height 
of which above the base line is equal to the load per square inch, its dis- 
tance from the origin measures the extension of the material as produced 
by that stress. 

(9. ) To determine the effect of a blow or a shock, whose measure is given in 
inch-pounds of energy, i. e. of which the work that it is capable of doing 
is known. 

Find a point on the curve whose ordinate cuts off an area between 
itself and the origin, representing the given amount of work. Or, find 
such a point that two-thirds the product of the stress measured 
by its ordinate, and the extension corresponding to its abscissa is 
equal to the number of inch-pounds given. The position of this point 
shows the maximum strain and the maximum extension of the material 
under the assumed conditions. Drawing a line through this point 
parallel to the nearest "elasticity line," the distance of the point at 
which it intersects the base line from the origin indicates the resulting 
set. 

(10.) To determine the effect of a blow upon the material, when already 
strained by a dead load. 

Determine first the extension produced by the application of the 
static stress, as in (8) , and then find that point on the curve between the 
ordinate of which and the ordinate of the point indicating the strain just 
found as due the dead load, an area is intercepted w^hich measures the 
work done by, or the energy of, the shock which has been assumed or 
calculated. 

15. Examples. — ^Illustrations of the first seven of the above described 
processes are either given in the preceding portions of this paper, or will 
be noticed hereafter during the progress of special researches. Those 
succeeding may be illustrated thus : 

(1.) Given, a load of 30,000 pounds j)er square inch. Determine its 
effect upon good qualities of cast and wrought iron, low steels, tool steel 
and the weaker metals, 

Keferring to Plate II for examples, we find that neither cast copper, 
h^ad, tin, nor zinc would sustain such strain; all would bo broken. 

Good irons, Nos. 1 and (5, would be strained beyond their limits of 
elasticity, and would take a set after an extension t)f about 1 and 1} per 
cent., resi)ectively. Tlie excei)tional iron. No. 22, would be strained to a 



point whicli is so nearly its elastic limit tliat it would remain i^racticallv 
uninjured. 

The low steels, Nos. 69, 67, 76, would bear the stress \\T.th a similar 
degree of safety, very nearly. The first would have a considerable margin 
of safety within its elastic limit; No. 67 would be nearly, and No. 76 would 
be quite, strained to the elastic limit, while No. 98 would take a set of 
about one-fifth of one -pev cent. 

If the strain were torsional, the weaker metals would be twisted off by 
a force corresponding to that here assumed, the good irons would take a 
set of about 25", the best iron and the three stronger steels would take 
no appreciable set, and the softest of the latter would set at about 10^. 
In these cases the specimens are supposed of standard size. For other 
sizes the forces producing similar effects would vary as the cubes of the 
diameters. 

(2.) Given, the magnitude of a shock, or blow, e. g. as equal to that 
due a weight of one ton, 2,000 pounds, falling one foot, the rod taking 
the strain being of one square inch area of section and one foot long. 
Determine the effect for each of the above-named materials. 

The effect of this blow is equivalent to an expenditure of energy 
amounting to 2,000 X 12 = 24,000 inch-pounds. 

The weaker materials, not possessing an ultimate resilience of this 
amount, would be broken. 

Forged coj^per would be strained and would take a set after veiy 
nearly 12y per cent, of extension, since 

0.12i X 12 X ^^'QQQ X 2^24,000 
o 

the work done by the blow being equihbrated by the product of two- 
thirds the resistance, noted at 110^, Plate 11, into the extension. Per- 
fect accuracy of flgnires may be insured by perfectly accurate measure- 
ments. 

The specimen of iron No. 1 would be given an extension and set of 
nearly 0. 068, since the resistance, under this amount of stretch, would be 
approximately 45,000 pounds per square inch, and the work during ex- 
tension would be 

0.068 X 12 X ^^'QQQ X ^ = 24,000 inch-pounds. 

The iron of special gTade No. 22 would be elongated 0.058 = 0.69 
inch, as 

0.069 X 12 X ^'^'QQQ X ^ = 24,000, nearly. 



6 

The same blow would produce on the rod, if made of such steel as 
No. 69, an extension of 0.0384 >( 12 = 0.461 inches, estimated thus, 

+ = 24,000 — 1.78,000 = 0.461, 
it being found by "trial and error," that the extension 0.0384 developes 
a maximum resistance of 78,500 x^ounds per square inch. 

It is evident that, where extreme accuracy is required, the curves 
should be transferred to a new scale in which the abscissas should be a 
scale of elongation instead of angular distortion, and the area should be 
carefully measured. '^ For the latter work an Amsler "Planimeter"! is 
useful. 

(3.) Given, a bolt of dimensions as last assumed, strained with the 
effect of a load of 30,000 pounds, as in example (1), determine the effect 
of a blow of 24,000 inch-pounds energy, occurring while the bar is sus- 
taining the static load. 

The effect of the dead load, as already calculated, is to x^roduce a 
strain upon the low steels, and upon iron like No. 22, which would keep 
them extended only a very minute fraction of their original length, this 
extension being, even with the latter material, but 0.05 of one per cent. 
The effect of the blow would be, practically, the same as has just been 
estimated for the unloaded bar. 

Nos. 1 and 6 would be, as already shown, extended 1 and It per cent. , 
respectively, by the simple load. The added effect of the blow would be 
to produce an additional extension and set of 0.0533 and 0.0555 resjiec- 
tively, since the mean resistance, during this extension, is 
45,000 4- 35,000 ^^^ 42,0 00 + 30^000 
2 ^^ 2 

respectively, and the extension must be, 

24,000 - 45,000 + 30,0 00 ^ 12^0. 0533 and 

2 

24,000 - ^00+30^^00 ^ 12= 0.0555 

2 

The bar is stretched in the first case 0.64 inch ; in the second 0.666 inch, 
by the blow, if made of such iron as that of specimens Nos. 1 and 6. 

It should be remarked here, that although the diagrams obtained 
from the various materials tested, give data from which to estimate their 
relative value in resisting shock, the absolute results of calculation, with 
no modification for varying rapidity of action, will be but approximate. 

* See London " Engineer," Nov. 8tli, 1872. 

t It is evident that diagrams accurately representing tests made with the common testing 
machine afford similar facilities for solving these problems. 



This is a consequence of the facts that the inertia of the body struck 
^vill affect the result, and that the actual resistance varies Avith the velo- 
city of rupture. A rod which will sustain safely the blow of a heavy 
body, would yield readily under a blow of similar energy struck by a 
light weight moving v.ith joroportionally increased velocity. The mathe- 
matical investigation of this effect, Avhich has not hitherto been noticed, 
remains to be given. It is only necessary to state here that a rod of 
uniform section, and homogeneous in structure, will be uniformly ex- 
tended by a force slowly ai3i3lied. A blow received at one end will extend 
it most at the portion nearest that end, and the more rapid the blow the 
more is its effect concentrated. It is possible to produce actual fracture 
at one end by a very rapid blow, and for rupture to become complete 
before the shock is felt at the opposite end. This action is seen daily in 
every workshop where pieces are broken from heavy masses by the blow 
of a hammer. 

The effect of a blow depends, therefore, not only on the magnitude of 
that product of mass into height due its velocity which we call vis viva, 
but also ui)on the magnitude of the factors. It further follows that of 
two materials having equal tenacity and equal ductility, the one having 
greatest density wiU be most liable to fracture by impact.^' This infer- 
ence is confirmed by experience. 

In general a rod should be somewhat larger at the end receiving the 
shock, and this enlargement should be greater as the blow is more rapid. 
Conversely, blows of equal energy are the most injurious when given by 
bodies of light weight^ jnoving at high speed. This difference is exagger- 
ated by any cause which increases the density of the material. The 
variation of resistance with the rapidity of rupture will be considered 
more at length hereafter. 

It is readily seen that we have here an explanation of the fact, that 
f ractiu'e produced by a quick blow is granular in character, while a steady 
l)ull brings out the "fibrous" texture of iron. In the former case the 
action is concentrated ui^on a cross section close to the point at which 
the blow is received ; in the latter instance inertia acts less effectively in 
resisting the transmission of the rupturing force to other portions of the 
piece, and the di*awing out process is permitted to take j)lace. 

16. PECiJiiiAE PeobijEms sometimes present themselves in practice 
which cannot be solved by any published methods — why, it is difficult to 
say, but partly, it is probable, because of a deficiency of experimental 

* Mechanics' Mag., Dec, 1871, p. 492. 



8 

data, and partly because known authorities have not been led by actual 
experience in engineering practice to perceive the importance of their 
applications. 

Of these the following is an example : 

To determine the effect of a succession of stresses, ivhetlier static or dynamic, 
each of which strains the material beyond the original or the acquired limit 
of elasticity. An illustration of this action is given by the repeated bend- 
ing, stretching, or other form of distortion by external force, of any 
materia] producing at each application a new set. The same case is illus- 
trated by the gradual elongation of a rod by repeated blows, the energy 
of each of which exceeds the elastic resilience of the material. 

Determine the elastic resilience of the material existing previous to 
the application of each stress, by taking the area comprised between two 
lines drawn through that point on the curve of the material chosen, 
whose abscissa represents the existing extension, one of which Hues is an 
ordinate and the other of which is parallel to ' the nearest ' ' elasticity 
line." This area represents the elastic resilience of the piece; i. e. a 
blow having an equivalent energy would leave the piece uninjured and 
without set. Deducting this amount from the energy of the given blow, 
the remainder of the work done by that blow is expended in producing 
set or extension, and may be determined as already described. 

The effect of a simple force may be determined by deducting from 
the total distortion producted by each application of that force, the 
elastic range of the material. 

It is thus readily ascertained, in either case, how much each applica- 
tion will add to the set, and how many applications will be required to 
produce rupture. It is here assumed that distortion within the elastic 
limit leavs the x^iece uninjured, however often it may be repeated. This 
assumption seems correct, a priori, and is well sustained by the valuable 
researches of Wohler"^" and others, f 

The effect of repeated bending, or other form of strain, can thus be 
inferred from ail examination of the strain diagram of the material, ob- 
taining from a single experiment a determination hitherto only obtained 
by a long and tedious process of repeated distortion. Such investigations 
of the "fatigue" of metals are often of great importance. 

17. The Effeot of Time on Metals left under Strain. — The effect 



* Zeilschrift fur Bauwesen, 1800;, Festiglieitversucke mit Eisen und StaJil, Berlin; also Loud. 
Engineering, 1871. 

t Fairhairn : CivU Engineer and Architects' Journal, Vols. XXIII, XXIV. 



9 

of stress is modified when metals are left under strain for considerable 
intervals of time. It had generally been suj)posed that tliis effect was to 
weaken the resistance whenever the material was left exposed to a strain 
exceeding the elastic limit. 

This idea seems sustained .by the experiments of M. Vicat, made at 
Paris about forty years ago.* He states that four wires were extended, 
respectively, by j, i, 4 and ^ their ultimate resistance, and their elonga- 
tions were observed and recorded at intervals of one year. The relative 
extensions observed indicated a gradual lengthening of the three which 
were strained bej'-ond the elastic hmit, and that most strained finally 
broke, after sustaining f its original ultimate breaking weight two years 
and nine months, the point of rupture being finally determined by the 
action of corrosion which had not been entirely prevented. 

The several extensions were as follows : 

No. 1, sustaining -}, 33 months . 000 per cent. 

No. 2, '^ h '• '■ 0.275 

No. 3, " -h " " 0.409 

No. 4, " I, " " 0.613 

The rate of extension was nearly proportional to the times, and the 
total extension to the forces. M. Vicat concludes that metal thus over- 
strained ^dll ultimately break, and his paper has caused much uneasiness 
among members of the profession, as indicating a i30ssibility of the ulti- 
mate failure of structures ha^aug originally an ample factor of safety. 

The elegant and valuable researches, also, of H. Tresca, on the 
fiow of solids, t and the illustrations of this action almost daily noticed 
by every engineer, seem to lend confirmation to the supposition of Yicat. 

The experimental researches of Prof. Joseph Henry, on the viscosity 
of materials, and which proved the possibility of the coexistence of 
strong cohesive forces with gi-eat fluidity, J long ago proved, also, the 
possibihty of a behavior in solids, under the action of great force, an- 
alogous to that noted in more fluid substances. 

On the other hand, the researches of the writer, as described in the 
first section of this paper, indicating by the form of strain diagTams, that 
the progress of this flow was accompanied by increasing resistance, and 
the corroborator^" evidence furnished by all carefully made experiments 
on tensile resistance, as those of King and Rodman, Kirkaldy and Styffe, 
made it appear extremely doubtful whether materials were really weak- 

* Annates de Chimie, et de Physique, 1834; Tome 54 ; p. 35. 

t Sur V Ecoulemeni des corps soHdes; Paris, 1869-72. t Proc. Am. Phil. Society, 1844. 



10 

ened by a continuance of any stress, not originally capable of producing 
incipient rupture. 

18. To determine this point, a series of experiments was made, the 
general result of which was first formally announced in a note to this 
Society,* in which the first experiment, commenced during the session of 
the National Academy of Science at the Stevens Institute of Technolog}', 
was described, and in which the first deductions, since slightly modified 
by an extended investigation, were given. In Plate III, No. 16, is a fnc 
simile of the strain diagram obtained at the first experiment. 

A piece of iron, of a good quality of metal, but badly worked, as 
shown in the sketch already given in Section 1, was placed in the 
machine and strained considerably beyond its elastic limit. It was then 
left twenty-four hours under the strain thus produced at A, Plate III. 
At the end of this period, the pencil was found precisely as it was left, 
and not the slightest evidence of yielding was noted. The slight depres- 
sion observed in many examples to be given, is produced by a slight 
compression of the wood used in blocking the machine at the beginning 
of the interval. No evidence of flow was therefore obtained. 

Upon attempting to produce further change of form, however, the 
unexpected discovery was made that the test-piece had acquired an in- 
creased resisting power. The pencil, instead of following the general 
direction taken the day previous, rose, as seen in the diagram, until a 
resistance was indicated, exceeding hy nearh/ 30 per cent, that shown be- 
fore the specimen was left under strain. This resistance having been 
overcome, the piece yielded with a slightly decreasing resistance, and. 
after considerable additional distortion, was left at B twenty-four hours. 
The result of the second experiment is seen to be an increase of nearly 
15 per cent. , and a third trial at G gave a small, but still perceptible, 
gain also. 

This singular phenomenon api^eared so remarkable and so important 
that experiments were continued upon various grades of iron, and 
upon other metals, the greatest care being taken to avoid any j)ossible 
source of error. Several strain diagrams are given illustrating some of 
these experiments. 

No. 10 represents that of a piece of good iron which is far more homo- 
geneous and better worked than No. 16. 

No. 68 is a piece of " Siemens-Martin steel" which was left under 



* Stt Trans. Am. Soc. C. E., Nov. 1873; Journal Franklin Institute; March, 1874. 



11 

strain, at A, twenty-four hours, and at B an equal length of time. In 
the latter case, the applied force was wholly removed, at the end of 
twenty-four houi'S, before an attempt was made to produce further change 
of form. On renewing the strain the resistance is seen to have acquired 
an increased intensity very nearly absolutely equal to that shown at A, 
and relatively gTeater, a fact which ^vill be found to aid in the determina- 
tion of the real character of the phenomenon. A third experiment, at 
C, shows a repetition of this action, and a fourth, similar to that at B, in 
all except time — for in the last experiment the time was but a fraction of 
an hour — gave a similar result. In each case it is noticeable that a slight 
fall from the maximum attained, follows the yielding of the test-piece. 

No. 33, malleable cast iron, No. 52, double shear steel, and No. 81, 
tool steel, aU exhibit this same stiffening under prolonged strain. 

No. 17, "homogeneous chrome iron," was subjected to experiment 
four times. At A, the effect is very marked, and the resistance to further 
change of shape continues to increase slowly until left at B for a second 
trial. The maximum attained at B is not sustained, as further distortion 
occurs, and, after a slight decrease, the specimen was again left, the 
pencil resting at C. 

Next day, the increase of resistance was found less considerable than 
at the previous experiment, and the line, after passing a maximum a few 
degrees beyond, falls quite rapidly. Fearing that the metal was about to 
rupture comi3letely, it was left once more at JD, another day, after which 
time its beha^aor was^ similar to that on earlier trials. It fully regains 
the maximum power of resistance noted after the trial at C, and, before 
rui^ture, it even slightly exceeds it. 

The hardest material experimented upon was the very hard chrome 
steel, No. 21. Left at A three days, the resistance at that degree of 
distortion was increased about 8 per cent., and, again at B, four days 
under strain gave a rise of nearly 4 per cent, after which a considera- 
ble rise occurred, in the ordinary manner, before rupture took place. 

An interesting experiment was made with the best Swedish iron, a 
metal of such wonderful purity and ductility that one specimen, of 
standard size, was twisted nearly 600° before completely breaking off. 

No. 101, Plate III, is the strain diagram of the specimen tested for 
the purpose of determining the effect of continued stress. Here, as 
seems frequently the case, a loss of ductility ax^parently accompanies the 
increase of resistance and the total resiUence appears to be comparatively 
slightly altered. 



12 

This specimen was strained until the limit of elasticity was just passed 
and was then left at A one day. The result, with even tlie slight distor- 
tion of but 6°, producing an extension of a very minute amount, 
is similar to that before noticed, and the behavior here exhibited prob- 
ably gives a clue to the causes of this peculiar action. After this trial, 
several others were made, and the metal is seen to have behaved in a 
manner precisely similar to the other grades of iron. 

19. Eeviewing all of the large number of experiments made since the 
discovery of this effect of continued strain, carefully comparing the 
curves obtained with each other, and with the diagi-ams obtained in the 
ordinary way, and, finally, making a comx3arison of the conclusions drawn 
from this research, with the results of the experimental work of other in- 
vestigators, the writer has been led to the following, as the most prob- 
able explanation of this singular molecular phenomenon. 

These strain diagrams are the loci of the successive limits of elasticity 
of metal, at successive ]30sitions of set. 

T he phenomenon here discovered is an elevation of the limit of elasticiii/ 
hy a continued strain. The cause is probably a gradual release of internal 
strain, occurring in a somewhat similar manner to that observed previ- 
ously in cast iron in large masses, and less frequently, and generally in 
a less marked degree, in wrought-iron and other metals, which have been 
worked in large pieces, and in which such strain has been more or less 
reduced by a period of rest.^' This loss of strength in large masses of 
wrought iron is stated, by Mallet, to amount frequently to one-half, f 

20. The manner in which this reduction of internal strain occurs, by 
continued stress at the limit of elasticity, as here observed, may be readily 
conceived. 

When the metal is thus strained, many sets of molecules are placed in 
positions in which they exert a maximum effect tending to produce mole- 
^cular changes which may equalize the originally irregular distribution of 
inter-molecular stresses. After a time, the change actually takes place by 
"flow," and the resisting power of the piece becomes increased, and its 
limit of elasticity raised, simply because its forces are now no longer 
divided, and may act together in resisting external forces. The diagram 
itself exhibits the best evidence of the occurrence of flow, but it is also 

* Compare London " Iron." Stability of Iron Structures, Feb., 1874; Van Nostrand's Maga- 
zine, April, 1874. 

t "On the Coefficients of Elasticity and Rupture in Wrought Iron in EelaUou to the 
Volume of the Mass, its metallurgic Treatment and the axial Direction of the constituent 
Crystals." Proc. Inst., C. K. 



13 

shown by an inspection of the broken specimen, as in Figures 6, 7 and 
11, of Section I. 

It was at first suspected that an action had been detected in this 
phenomenon, similar to that by which the "portative force" of a magnet 
is increased by loading its keeper more and more heavily until it becomes 
" supersaturated. " It is not impossible, perhaps not improbable, that the 
two cases are similar, in some respects, this behavior of the cohesive 
force in the i^resent example, aiding to produce the extraordinaiy increase 
of resisting power here observed. 

A comparison of the ductility registered by samples variously treated 
lends some confirmation to the supposition, formerly expressed, that, in 
all cases, the resilience does not increase in the same proportion as the , 
increase of mean resistance, as a consequence of sustained stress, and 
this, if a fact, may possibly be considered as corroborating the idea just 



The discovery of this elevation of the elastic limit in all metals 
examined is also probably confirmatory of this idea. 

If the explanation, just offered, of the apparent strengthening action 
of prolonged stress is correct, the conclusions of Vicat are not over- 
thrown, although evidently not fully justified by his own experiments, 
and although the intervening forty years of engineering practice have 
not produced evidence which maybe considered as confirmatory of them. 

The same molecular movement, or flow, which rearranges the internal 
forces and reheves internal strain, may be a phase of that viscosity which 
Yicat supposed might in time permit rupture of metal, subjected to 
stress nearly apjproaching its original ultimate resistance, the one action 
being a more immediate result than the other, and the latter producing 
its effect, even when cohesive force may have been actually intensified. 

* Since the above was written, the Journal of the Franklin Institute has, in the issue for 
March, 1874, given an account of experiments made by Com'd'r Beardslee, U. S. N., during 
which metal strained beyond the elastic limit by tension, exhibited a gain of resistance at a 
position of earliest set, i.e., an elevation of the elastic limit, from 23,075 to 26,100 pounds per 
square inch, 13.1 per cent., in seventeen hours. The material was bloom iron turned approxi- 
mately to one-half square inch section. 

A very important fact noted by the experimenter is that an apparent positive action was 
observed in the metal under strain by which the scale' beam was actually thrown up by a force 
measuring 125 pounds. If this observation is not erroneous, or, if the action was not due to 
some accidental circumstance, we may have here a measure of the intensifying effect above 
described. 

Commander Beardslee has communicated to the writer the experimental confirmation of 
the fact, deducible from the explanation here given, that this release of internal strain occurs 
to a nearly equal extent if the strained piece is simply laid aside for a similar interval after it 
has been given a set. 



14 

Should this prove to be the fact, it would seem allowable to conclude 
that the forces of polarity and cohesion are not identical. The cause of 
the apparent increase of strength of iron, with increase of temperature, 
is seen to be explained by this relief of internal strain, which occurs 
most readily at high temperatures. 

The experiments of the writer have not indicated the possibility of 
continued flow, and, consequently of ultimate rupture, except under 
stress, increasing in intensity up to the full maximum resistance of the 
material.* They do not, therefore, confirm Vicat's deductions, and the 
inference would seem, on the contrary, to be — that structures of metal 
do not become weakened with age, except as injury occurs by corrosion, 
or by overloading. The experiments of Boeblingf and his opinion, as 
expressed in his rei3ort on the Niagara SuiDension Bridge, are apparently 
correct. 

Kirkaldy, also, concludes that the additional time occupied in testing 
certain specimens of which he determined the elongation ' ' had no in- 
jurious effect in lessening the amount of breaking strain, ".I An examina- 
tion of his tables shows those bars which were longest under strain to 
have had highest average resistance. 

21. Wertheim supposed that greater resistance was offered to rapidly, 
than to slowly, produced rupture ; Kirkaldy concludes that the contrary 
is the case. Eedtenbacher| and Weisbachy assume the law of resistance 
to be the same beyond the limit of elasticity as within it, and deduce 
formulas for resistance to shopk which are widely inaccurate. 

The experiments of the writer prove that, as had ah'eady been indi- 
cated by Kirkaldy (whose results, however, had been looked on by many 
of the profession with some suspicion), a lower resistance is offered as the 
stress is more rapidly applied. This conspires with vis viva to produce 
rupture. 

This is seen at w, No. 101, Plate III, where a sudden increase of velo- 
city produced a depression of the line, at the angle 110^, and it is 
exhibited in a much more marked degree in No. 118. 

In the latter example, the strain was gradually applied until the point 
a was reached, when, with a suddenly applied force, a motion estimated 
at about one-tenth of a foot per second, was obtained and, immediately 
at b, the resistance feU off very considerably, the pencil droi3ping to c. 

* Compare Kirk&ldy— Experiments <m Wrought Iron and (Sieei— pp. 62-69; also see the plates 
given by Styfife, exhibiting curves for tension. 

t Journal Franklin Institute, 18C0; Vol. XL, p. 360. § Der Maschimenhau, Vol. I. 

t Experiments on Wrouyhi Iron and Steel, pp. G2, 83. || Mechanics of Enginceriiuj, etc. 



15 

Again resuming the slowest moyement, about one liundi-edtli of a foot or 
less per second, resistance rose again to d. A repetition of the rapid 
movement between d and h^, was followed by a loss of resistance again 
from h^ to c^, and, as is seen by the diagram, this occurred whenever the 
exi^eriment was repeated. A.t k, distortion hj a very slow movement was 
resumed, and continued until the siDecimen broke. Here we have prob- 
abh^, the first direct determination of this question, in which the effect 
of vis viva does not appear. 

We may, therefore, conclude that the rapidity of action, in cases of 
shock, and where materials sustain live loads, is a very important element 
in the determination of their resisting power not only for the reason 
given ah-eady in paragraph 3 of this section, but because the more rapidly 
the metal is ruptured, the less is its resistance to rupture. This loss of 
resistance is about 15 per cent.^ in No. 118. 

The cause of this action we may presume to bear a close relation to 
that operating to j)roduce the opposite phenomenon of the elevation of 
the elastic limit by prolonged stress, and it may probably be simply 
another illustration of the effect of internal strain. 

With a very slow distortion, the ''flow" already described occurs, 
and but a small amount of internal strain may be produced, since, by the 
action noticed when left at rest, this strain relieves itself as rapidly as 
produced. A more rapid distortion produces internal strain more rapidly 
than relief can take place, and the more quickly it occurs, the less 
thoroughly can it be relieved, and the more is the total resistance of the 
l^iece reduced. Evidence confirmatory of this exiDlanation is found in the 
fact that bodies most homogeneous as to strain exhibited the least of 
these effects. 

It does not seem impossible that, at extremely high velocities, the 
most ductile substances may exhibit similar behavior, when fractured by 
shock, or by a suddenly applied force, to substances which are really 
comparatively brittle. 

In the production of this effect, which has been frequently observed 
in the fracture of iron, although the cause has not been recognized, the 
inertia of the mass attacked and the actual depreciation of resisting power 
just observed, conspire to x^roduce results w^hich would seem quite inex- 
plicable, except for the evidently great concentration of energy here re- 
ferred to which, in consequence of this conspiring of inertia and reduced 

* Compare Kirkaldy, p. 83, where experiments, wMch are possibly affected by the action of 
vis viva indicate a very similar effect. 



16 

resistance, brings the total effort upon a comparatively limited portion of 
the material, iDroducing the short fracture with its granular surfaces, 
which is the well known characteristic of sudden rupture. 

Any cause acting to produce increased density, as reduction of tem- 
perature, evidently must intensify this action of suddenly applied stress. 

The liabihty of machinery and structures to injury by shock is thus 
greatly increased, and. it is quite uncertain what is the proper factor of 
safety to adopt in cases in which the shocks are rapidly produced. This 
uncertainty must remain until further experiment shall be made the basis 
of a correct mathematical expression of the natural laws involved in the 
problem. 

Meantime the i3recautions to be taken by the engineer are: To pre- 
vent the occurrence of shock as far as possible, and -to use in x)arts ex- 
posed to shock, light and elastic members, composed of the most ductile 
materials available, giving them such forms as shall distribute the distor- 
tion as uniformly and as widely as possible. 

22. The behavior of materials subjected to sudden strain is thus seen 
to be so considerably modified by both internal and external conditions, 
which are themselves variable in character, that it may still prove quite 
difficult to obtain mathematical exiDressions for the laws governing them. 
It is not improbable, however, that an approximation, of sufficient 
accuracy for all cases which frequently arise in practice, may be obtained 
by a study and comparison of experimental results obtained, as above, by 
the method here adopted, and one which seems peculiarly adopted to 
the work. 

A carefully conducted series of experiments giving quantitative results 
would be of great value. Without such a research no reliable knowledge 
can be obtained of the law of depreciation, and no useful formulas can be 
devised for use in calculation. The experiments made by the writer are 
.not yet sufficiently numerous or precise to serve as data from which to 
deduce equations. 

23. The Elasticity of the Metals. — The examination of the " elas- 
ticity line " will be found to present some facts of interest. 

It will be seen that, in every case, the line produced by the descent of 
the pencil is not precisely coincident with that formed by its rise. This 
is not due to the friction of the machine, as that would not cause the de- 
cided difl'erence observable in the form of the curve. It may be partly 
due to the fact that the set produced is partly temporary.* 



*Moriii, Resislance des Materiaiix, p. 10. 



17 

An attempt was made to determine its law by tlie following method: 

A steam gauge liaving a recording apparatus, in which the paper was 
moved horizontally, at a uniform rate, by a well-constructed clock, was 
kindly furnished by the Messrs. Edson, of the New York Recording- 
Gauge Co, 

This was set up by the side of the machine, upon a stand on which 
could be clamped the test piece. The latter carried a light, long pencil 
holder, so aiTanged as to traverse the paper in a direction at right angles 
to that of the motion given it by the clock. An angular motion in the 
specimen of 0.05° would be readily observed. The specimen was 
given a degTee of torsion of from 10° to 360°, in different cases, and was 
then rapidly transferred to the stand, where the restoration of form 
would record itself upon the moving paper, forming a curve of which the 
ordinates would represent the restoration of form, and the abscissas 
would be x^roportional to the time. 

In all cases observed, the restoration of form, by loss of temporary 
set, was so rapid as to have become completed before the specimen could 
be placed in the recording apparatus, and the record made a straight line 
invariably. 

The conclusion which has finally been deduced from a study of the 
diagTams obtained is, that the peculiar feature here alluded to is a conse- 
quence of an internal molecular friction, the existence of which has 
already been long suspected by the writer, and probably by many other 
experimenters. 

An illustration of a similar action probably occurs in the behavior of 
iron under magnetic action. Magnetization produced in bars of various 
qualities of iron and steel during important researches of Dr. Joule* and 
Prof. Tyndal,t proved this behavior to be common to all, when the 
change of form was produced, within a very minute range even, by mag- 
netic force. Dr. Mayer has more recently X examined this peculiar form 
of molecular action with great skill and thoroughness. Its existence is 
undoubtedly well proven, and the lines, above referred to, on the strain- 
diagrams seem to exhibit its effect very clearly. Possibly the rise from 
/ to g, No. 118, Plate HI, is due to such friction. 

24. The evident proof found, in the parallelism of all elasticity Unes 
in each diagTam, of the fact, first noted by some of the earhest expeii- 

* Philosophical Magazine; 1874. t Researches on Diamagnetism, etc., 1870. 

+ " Effects of Magnetism in changing dimensions of iron and steel bars," by A. M. Mayer, Ph. 
D.; Steven's Instit-ate of Teclinology; 1872. American Journal of Science and Arts; 1873. 
<2 



18 

menters in this field, that elasticity remains quite unimpaired up to the 
point at which rupture commences, has been already adverted to. 

Coulomb describes a series of curious and instructive experiments 
which may assist in determining the molecular action occurring in these 
instances where great distortion and great permanent displacement of 
particles takes place without loss of elasticity."^ 

He found this to occur, not only with metal wires, but T^^.th threads of 

line clay, ^2 of an inch in diameter and 11 feet long, which could be 

* 

twisted 5-^ turns repeatedly without set and without apparent loss of 
elasticity. Turning the thread through a wider range of torsion, it 
always returned but 5^ revolutions, and in each new position of set ex- 
hibited the same elasticity as before. 

The explanation of this action, as illustrated by the strain diagrams of 
Plates II and III, and by Coulomb's experiments, is probably also to be 
found in the phenomenon of flow of solids. The restoration of cohesion, 
in bodies actually separated, exhibits the extent to which this action may 
proceed. Two freshly cut surfaces of lead when brought together, with 
a moderate pressure cohere firmly, and plates of glass, laid one upon 
another, sometimes "seize" each other so firmly that they are cut and 
worked as one piece, f The welding of iron is another and a very familiar 
illustration of the same action. Cohesion may therefore be actually de- 
stroyed and renewed, and molecules may move among each other, changing 
completely their relative positions, without loss of either strength or 
elasticity. 

The result of these experiments on metal are important as exhibiting 
the error of the opinion hitherto entertained by many physicists and en- 
gineers, among whom was the writer, J that straining metal might weaken 
it, even when rupture did not commence, and even where no condition of 
internal strain was induced. It has been here shown that elasticity re- 
mains unimpaired, and resistance continually increases up to the point 
at which incipient fracture takes place. No well proven exception to 
this law has been observed. 

25. While comparing the inclination of the elasticity lines vrith. the 
initial line, to determine the pressure and the amount of internal strain, 
it has been noticed that more or less strain seems almost invariably to 
exist, but that the amount, as indicated by the difference in inclination of 

* Lecture Notes on Physics; Mayer. Journal PranMin Institute, 1868. 
■\ Miller: Chemical Physics; p. 67. 
+ Journal Franklin Institute. 



19 

the two lines, is not always as well shown by the greater or less curvature 
of the initial portion of the diagram. The ^Drobable reason would seem to 
be that this strain is not always uniformly distributed. Were the strain 
considerable and uniformly distributed, the initial line would be strongly 
convex toward the base line, and would have the parabolic form. Absence 
of strain is indicated by a straight line rising regularly to the elastic limit, 
or, in many cases where the elastic limit is very low, and when the 
material is inelastic and flows without tendency to recoil, concave toward 
the base, and parabolic. Irregularly distributed strain would modify the 
parabolic cm-ve, and the amount of strain would determine the total 
curvature. 

The initial and elasticity lines have, therefore, gTeat interest as reveal- 
ing important and other^^ise unrecognizable properties of the material. 

It has been remarked that the difference of inclination just referred 
to, proves the truth of the assertion of Hodgkinson that every load pro- 
duces a set. It can now be readily seen why this should usually be the 
fact, and also that, although it is true, it does not necessarily indicate 
injury of the material. 

Since, in its ordinary state, many sets of particles are usually in a 
condition of maximum^strain, the slightest application of external force 
to the piece will destroy the existing equilibrium among these conflict- 
ing forces within the mass, producing a change of form, and either 
rupturing or producing a flow of those particles which are most strained, 
and thus causing a new condition of equihbrium, the piece returning 
only approximately to the original form when relieved. The gTeater or 
less the applied force, the greater or less the number of displaced par- 
ticles, but it is only when the set becomes nearly proportional to the 
distortion that it assumes the character in which it is looked upon as a 
serious efl'ect. 

With perfectly homogeneous materials, free from internal strain, no 
such action would be noticed, and the earliest set would occiu- beyond 
the elastic limit, which limit is here considered to be attained when the 
set becomes proportional to the distortion. 

26. The very minute range of distortion within the elastic hmit is 
shoT^m by the strain-diagrams very beautifully. This j)oint is usually 
reached A^ith the first 5^ of torsion, and where internal strain has been 
eliminated it is frequently found mthin 2°, the corresponding extension 
being much less than .0001. 



20 

Captain Rodman, who lias made the most delicate determination yet 
published,-^ detects a set of 0.000,001,4 after an extension of 0.000,27 
in a specimen of cast-iron, and the elastic limit, as here defined, is 
reached after an elongation of about 0.0003, the point not being, how- 
ever, very accurately determinable on account of the insensible change 
of rate, as akeady observed in the strain-diagrams of cast-iron. 

The immense magnification, on the strain-diagTams, obtained with 
the torsion machine, of the elongation at the commencement of the 
curve enables the behavior of the materials within this minute, yet most 
important, portion of the entire range to be perfectly rexjresented, and 
permits its examination in a most satisfactory manner. 

27. The Influence of Vabiations of Tbmpekatuee. — The efi'ect 
upon the mechanical properties of metal of variations of temperature 
has long been a subject of debate, and one which has not even yet been 
satisfactorily settled by experiment. 

A priori it would appear that, in a perfectly homogeneous material, 
entirely free from internal strain, change of tem^^erature would produce 
an alteration of strength and of ductility which would both be the 
reverse, in direction, of the variation of temperature. 

The forces acting to produce mechanical changes being, probably, 
cohesive force, on= the one hand, resisting external forces tending to pro- 
duce distortion or rupture, while the force produced by the energy of 
heat motion conspires with external force to produce that distortion, and 
the molecules being, at every instant, in equilibrium between the force 
of cohesion, on the one side, and the sum of the other two forces men- 
tioned, on the other, variations of form must ensue with every change in 
the relative magnitudes of these forces. A change of temperature pro- 
duced by an increment of heat energy, it would appear, must produce a 
reduction of cohesion by separation of particles, and the opposite 
change must cause an increase of cohesion by their approximation. In- 
crease of temperature, by reducing the range of action of cohesion by 
separating particles, and causing them to approach the limit of reach of 
cohesive force, would reduce ductility, and the opposite change of tem- 
perature would increase extensibility. The effect on resihence, the pro- 
duct of ductility and strength, would evidently be stiU more marked 
than the variation of its factors. 

The peculiar behavior of zinc, and the often observed brittleuess of 



* Experiments on Meialsfor Cannon, etc., Kodman, pp. 157-167. 



21 

iron, at low temperatures, have given cause for doubting the truth of the 
above statement, and until the j)henomena accompanying variations in 
homogeneousness of structure and composition, and the introduction or 
removal of internal strain, have been very thoroughly investigated, it 
cannot be anticipated that the subject will become well understood. The 
character of polarity, that force of which the presence constitutes the 
disting-uishing difference between solids and liquids, remains to be deter- 
mined, and its determination may be expected to throw important light 
upon this subject. 

Experiments of both i^hysicists and engineers have failed, up to the 
jDresent time, to give as much, and as precise information as is needed 
to determine satisfactorily what rules should govern the proj)ortions of 
structures, whether carrying dead loads or subjected to shocks or blows, 
at any given temperature below the usual range, or even at the low tem- 
peratures to be met during every winter in the latitude of New York. 

In a paper recently published* on "Molecular Changes produced by 
Yariations of Temperature," the \\T.iter gave the results of a careful in- 
vestigation of the experimental work previously done, by both philoso- 
phers and engineers, in researches bearing upon this important question. 

28. The conclusions, as there reached,- were the following: 

(1.) That the number and the nature of those molecular forces which 
determine the physical condition of matter are not yet fully ascertained, 
but that these forces manifest themselves in, at least, three distinct modes 
of action, and, as thus exhibited, they are known as repulsion, cohesion, 
and polarity. 

(2.) That the force of rei^ulsion is, apparently, heat motion, or some 
closely related i)hase of energy. That the force of cohesion bears some 
resemblance to that of gTavitation, but seems not to be identical with the 
latter, and that the force of molecular polarity, which determines the 
molecular relations of position, seems to bear some distant relation to that 
of magnetic i)olarity. 

(3.) That the law which governs the variations in intensity of these 
forces with changes of intermolecular distances, is undetermined, and that 
it has not been expressed by any mathematical formula, except ap]3roxi- 
mately and for a limited range. 

(4.) That the magnitudes of the intermolecular spaces, and, conse- 

* Iron Age, June, 1873; Van Xostrand's Mag., July, 1873; Jour. Frank. Inst., 1873; London 
Jour., Jan., 187J:. Also in pamphlet, p. 29. D. Van Nostrand. 



22 

quently, tlie volume of any mass, are variable with changes in the relative 
magnitudes of the forces of cohesion and repulsion. 

(5. ) That the resistance offered to change of form is determined by the 
relations in intensity of the forces of polarity and those forces which de- 
termine intermolecular distances. 

(6.) That, at the "absolute zero" ( — 461.2'^ Fahr.), cohesion, and 
consequently the strength, of the material has, probably, its maximum 
value, heat-energy having disappeared. 

(7. ) That, at very high temperatures, heat-energy exerts a separating 
force upon particles, which entirely overcomes the other forces, and 
matter, assuming the gaseous state, requires the action of extraneous 
forces to preserve its volume unchanged. 

(8.) That, at intermediate points, matter, in either the sohd or the 
liquid states, exhibits a definite degree of separation of molecules which 
is determined by the intensity of the repulsion due to heat motion, a 
position of equilibrium being assumed, which, with the same substance, 
is invariable for the same temperature. The application of some kind of 
force is required to disturb this equilibrium and to produce change of 
volume. The amount of this force is determined, for any given extent 
of disturbance, by the maximum value of cohesion for the substance and 
the quantity of heat which has been required to raise it from the absolute 
zero of temperature. 

The sum of the applied force and the force consequent upon the 
presence of heat motion must exceed cohesive force to x^roduce dilatation, 
while this cohesive force, added to externally applied force, must exceed 
the force of repulsion to produce diminution of volume. 

(9. ) That the distinction between the solid and liquid states of matter 
seems due to the action, in the former, of the force of polarity, which 
gives stability of form, while, in the latter, this force is extremely feeble, 
disappearing altogether when the boundary line between the liquid and 
gaseous states is reached. 

That combined stability and elasticity of volume may he produced by 
the equilibrium of attractive and repulsive forces, but that stability and 
elasticity of form demand the coexistence of cohesion and polarity. 

(10.) That the general effect of increase or decrease of temperature is, 
Avith solid bodies, to decrease or increase their power of resistance to rup- 
ture, or to change of form, and their capability of sustaining "dead''' 
loads. 



23 

(11.) That the general effect of change of temperature is to produce 
change of ductiUty, and, consequently, change of resilience, or power of 
resisting shocks and of carrying " live " loads. This change is usually 
opposite in dii*ection and gTeater in degTee than the variation simultane- 
ously occurring in tenacity. 

(12. ) That marked exceptions to this general law have been noted, but 
that it seems invariably the fact that, wherever an exception is observed 
in the influence upon tenacity, an excexDtion may also be detected in the 
effect upon resilience. Causes which iDroduce increase of strength seem 
also to produce a simultaneous decrease of ductility, and vice versa. 

(13. ) That experiments upon cop^Der, so far as they have been carried, 
indicate that, (as to tenacity), the general law holds good with that metal. 

(Itt.) That iron exhibits marked deviations from the law between ordi- 
nary temperatures and a point somewhere between 500^ and 600° Fahr. , 
the strength increasing between these limits to the extent of about 15 per 
cent, with good iron. The variation becomes more marked and the 
results more irregular as the metal is more imxDure. 

(15.) That above 600° F. and below 70°, the general law holds good 
for iron, its tenacity increasing ^ith diminishing temperature below the 
latter point, at the rate of from 0.02 to 0.03 -per cent, for each degTee 
Fahrenheit, while its resihence decreases in a higher but not well deter- 
mined ratio for good ii'on, and to the extent of reduction to one-third its 
ordinary value, or less, at 10° F. when cold short, and, in the latter case, 
the set may be less than one-fourth that noted at a temperature of 84° 
Fahr. 

(16. ) That the viscosity, ductility and resilience of metals are deter- 
mined by identical conditions, and that the fracture of iron at low tem- 
peratures has, accordingly, been found to be characteristic of a brittle 
material, while, at higher temperatures, it exhibits the appearance pecu- 
liar to ductile and somewhat viscous substances. The metal breaks in the 
first case, with shght permanent set and a short gTanular fracture, and in 
the latter with frequently a considerable set and a form of fracture indi- 
cating great ductihty. The variation in the behavior of iron, as it ap- 
proaches a welding heat, illustrates the latter condition in the most com- 
plete manner. 

(17.) That the x^recise action of the elements with which iron is liable 
to be contaminated, and the extent to which they modify its behavior 
under varying temperature, remain to be fully investigated, but that the 
presence of xDhosphorus. and of other substances producing " cold short- 



24 

ness," exaggerates, to a great degree, the effects of low temperature in 
producing loss of toughness and resilience. 

(18, ) That the modifications of the general law with other metals than 
iron and copper, and in the case of alloys, have not been studied and are 
entirely unknown. 

(19.) That these conclusions are sustained by experiments of both 
physicists and engineers. 

' ' The practical result of the whole investigation is that iron and cop- 
per, and probably other metals, do not lose their power of sustaining 
' dead ' loads at low temperatures, but that they do lose to a very serious 
extent their power of sustaining shocks or of resisting sharp blows, and 
that the factors of safety, in structures need not to be increased in the 
former case, where exposure to severe cold is apprehended, but that 
machinery, rails and other constructions which are to resist shocks, 
should have larger factors of safety, and should be most carefully pro- 
tected, if possible, from extreme temperatures." ' 

29. The conclusions above given were deduced from the physical in- 
vestigations of Boscovitch, Coulomb, Henry, Powell, Cagniard de la 
Tour, Andrews, Faraday, Wartman, Robison, Gaudin, Thomj)son, Ean- 
kine and others, and from the more purely technical researches of Pro- 
fessors Johnson and Norton, Pairbairn, Kirkaldy, Brockbank, Joule, 
Spence, Styffe and Sandberg. 

An apparent discrej)ancy of results from which some expeiiments 
seemed to indicate weakness, and others strength of metals as a conse- 
quence of reduced tem^Derature, was explained by the fact that those 
which seemed to prove the first conclusion, were cases in which the metal 
was tested by blows, and those proving the reverse were tests made by 
steady strain. The deduction (11), made above, that the result of change 
of temperature, by producing changes of ductility, which were the 
reverse of those produced in tenacity, and that the same bar might thus 
exhibit a higher resistance to static stress while less capable of resisting 
shock, explained the seeming contradiction. 

30. It was evident that, to determine satisfactorily the real effect of 
changes of temperature, it was necessary to obtain a series of experi- 
mental determinations of the simultaneous action of such variations upon 
both strength and resilience. Such experiments could readily be made 
by the method here pursued, and a (considerable number of observations 
are represented by strain-diagrams on Plate III. 

In these experiments, th(^ machine and tlu^ test pieces were })LK*od in 



25 

the oi3eu air where, changing in temi3erati"ire with the atmospheric 
changes, no error could arise by transfer of heat dming the experiments. 
The machine and the specimens submitted to test were always of the same 
temperature. 

The mildness of the past winter has precluded the determination of the 
behavior of iron at temperatures very far below the freezing point, the 
lowest reached being 10° Fahr. 

This is the more to be regTetted since, as will be seen, there exists a 
possible change of law near the Fahrenheit zero, and it is extremely im- 
portant to ascertain whether this indication of an anomaly arises from 
irreg-ularity in the quahty of specimens, nominally of the same grade, or 
whether it is a real variation of the effect of change of temperature. 

As no previously made experiments combine, in the manner here x^re- 
sented, the various effects of heat up>on the mechanical properties of the 
metals, the results obtained are given as a beginning, and the conclusions 
which are deduced from them are given as merely probable, while it is 
to be hoped that other members of the profession, who may be so situated 
that they can readily continue the work at points in the north and north- 
west where a temperatiu'e far below zero is reached, will make more com- 
plete and instructive researches during succeeding winters. 

It is apparently quite impossible to avoid error if the attempt is made 
to experiment with specimens cooled down by freezing mixtures, and the 
writer would only feel justified in presenting the results of out-of-door 
work. 

31. Eef erring to Plate III, the strain-diagrams of the best, and of 
medium tool steel, of German and double shear, of the several grades of 
iron and of copx)er and bronze are given, for temperatures from 70^ down 
to 10" Fahr. A diagram is also given in which the horizontal scale of the 
plate is taken to represent absolute temperatures on a one-fourth scale, 
and at ordinates representing respectively 10° 18°, 25^ and 70°, the 
resistances of the several specimens are laid off, and dotted lines connect- 
ing them indicate the rate of variation of strength with temperature. 

It will be seen that with the single exception of a scrap gray cast iron, 
presumably unusually imj^ure (IS'os. 25, 26), the effect seems invariably 
to have been a simultaneous increase of both strength and ductility with 
decrease of temx^erature to 18° and usually to 10° from 70° Fahr. In the 
case of the cast-ii'on, the increase of tenacity and reduction of ductility 
at the lower temperature are equally well exhibited, and the result is a 
slight decrease of resilience. 



26 

It will be noticed that the general trend of the lines in the diagram 
prepared for comparison of results, is very evidently towards a point 
on the scale (250°), corresponding to a temperature of 1000° above the 
absolute zero, at which point, were the diminutions proportioned to tem- 
perature, the metals would lose all cohesion. Since, however, the law, 
as determined approximately by the Committee of the Fi'anklin Institute, 
is expressed by a parabolic equation, the fact that their melting points 
are nearer 3,000°, or perhaps 4,000°, Fahr. above the absolute zero, does 
not conflict with the results of experiment. 

32. Comparing the several specimens of " good cast-steel," it is found 
that the four (Nos. 46, 47, 49, 50), whose strain-diagi-ams are given, 
evidently differ nearly as much in their individual properties as in their 
alteration by temperature. The two pieces tested at 70° Fahr. give a mean 
of strength, ductility and resilience, which is less than either of the other 
specimens. The strongest piece was broken at 18°, while that tested at 
10° is very nearly its equal in that respect, and is more than 10 per 
cent, better in extensibility and nearly 10 per cent, superior in resili- 
ence. The difference between the specimens tried at 10° and at 70° 
(average of the two) is about 15 per cent, in ductility and resilience, and 
rather more than 5 i^er cent, in tenacity. The piece tested at 10° has a 
Hmit of elasticity exceeding that of the specimens tested at 70° in about 
the same proportion. 

The double shear steels are irregailar, as would be expected from their 
method of production, but the greatest ductility is shown by the speci- 
men tested at 25° F., and the greatest tenacity by that broken at 25° also. 
The weakest is that tested at 70°, its loss of strength, ductility and resili- 
ence being very striking. The position of the elastic limit varied with 
that of the ultimate strength. " German " (English) steel exhibits great- 
est strength at 18° (No. 60), greatest ductility at 70° (No. 58) and greatest 
. resilience at 25° (No. 25) . 

"Medium crucible" steels seems strongest at 18° Fahr., most ductile 
and equally resilient at 25°, and weakest, least ductile and least resilient at 
70°, Nos. 78, 54, 70 are their strain-diagrams. 

Swedish irons (No. 99 and 100) were tried at 25° and 70°, and the 
result is again that the greatest resistance and greatest extensibility occur 
at the lowest temperature, the difference here amounting to something 
less than 10 per cent, at the elastic limit and about one-half as much at 
the maximum. 



27 

A piece of common iron was selected by tlie blacksmith from his stock 
as " the worst specimen of cold short iron in the shop." The two speci- 
mens, Nos. 130 and 132, were taken from the bar and tested at 10° and 
70° Fahr. respectivelY, with a result which is nnexpectedly similar to those 
already given in the variation of ultimate strength, dnctihty and resilience. 
The increase of strength at the lower temperature is apparently nearly 15 
per cent, the increase of ductiHty about the same, and the increase of re- 
silience 30 per cent. At the elastic limit this is reversed, however, the 
si^ecimen tested at 70" showing the highest elastic resistance. That 
broken at 10^ exhibits a very considerable amount of internal strain, to 
the presence of which may be attributed the exceptional behavior of 
these pieces. A similar difference but op^DOsite in direction and less in 
amount is noticeable in the Swedish irons, Nos. 99 and 100. 

Copper, Nos. 133 and 134, and bronze, Nos. 137, 138, both show a con- 
siderably greater strength at 10^ than at 70 ^ and a slightly improved 
ductility. The increase of tenacity and resilience amounts to 20 -per cent, 
and 30 i3er cent, respectively, the ratio being slightly greater at the limit 
of elasticity. 

Cast-iron exhibits the most striking increase of tenacity, the ratio of 
increase being above 30 per cent, and in ductihty having a mean value 
of 50 per cent. One specimen, No. 25 C, has a serious defect of inter- 
nal strain, but eliminating that by cutting off the sharply curved lower 
portion of the line, it would coincide very exactly with the companion 
specimen, Xo. 25 D, broken at the same temperature, 25°. 

33. It would seem, after a study of these experiments and after a com- 
pai-ison with those described by other exiDerimenters, that, although a con- 
siderable irregularity, due to differences in material nominally of identical 
character, tends to obscure them, that we may feel some confidence in 
drawing the following conclusions in modification and extension of those 
ah'eady given in article 28: 

(20.) That, with -pure, well- worked metals, the principle enunciated in 
article 28 is fully illustrated, and a decrease of temperature is accom- 
panied by increase of strength, ductility and resilience. 

(21, ) That materials which are impure and irregailar in character may 
exhibit exceptions to, and.even reversals of, that principle in changes of 
ductility and, while increasing in power of resisting simple stress, may 
lose their x^ower of resisting shock, by a diminution of temperature. 

We may hence feel confidence that, with really good iron or steel, we 



28 

are not exposed to seriously increased danger of failure of structures at 
such low temperatures as frequently occur in this latitude. 

There is no reason to believe that the familiar effect of phosphorus in 
" cold-shortening" iron and steel, when cold, has its maximum effect at 
ordinary temperatures. The experiments of Sandberg would seem to 
prove this effect to be intensified continually mth decrease of tempera- 
ture. 

34. Since the above research was concluded, the writer has become in- 
debted to the thoughtful kindness of Mr. Chas. Francis Adams, Jr. , for 
reports, one of which is that of the Mass. R. R. Commissioners for 1874 
containing (page 144, etseq.), the result of an investigation of the cause 
of rail breakage on a considerable number of railroads in the United 
States and Canada. The conclusions given are that ' ' cold does not 
make iron or steel brittle, or unreliable, for mechanical purposes," and 
that ' ' it was not the rule that the most breakages occur on the coldest 
days." The introduction of steel, in place of iron rails has caused an 
almost complete cessation of the breakage of rails (p. 150). 

The deduction which would seem proper in comparing these latter 
statements with the work of Sandberg would seem to be, simply, that 
the latter experimented with cold-short rails. The same conclusion is 
given in Sandberg's own statement of the difference between Welsh and 
French metal. ^' 

35. We still require, to give us reliable information regarding excep- 
tional cases, a series of experiments to determine the action of exces- 
sively low temperature, and whether the apparent change of law near 
the Fahrenheit zero is a natural or an artificial phenomenon. We need 
to learn precisely the effects of sulphur, phosphorus, and silicon at 
extremely low temperatures. We need, also, and especially, to learn 
by experiment whether extremely low temperatures occurring during 
our winters, produce a serious effect upon iron and steel by the intro- 
duction of internal strain as the material decreases in volume and in- 
creases in density. 

The uncertainty still existing as to the extent to which increased den- 
sity at low temperatures and reduction of tenacity under sudden strains 
at all temperatures — phenomena which have been revealed but not meas- 
ured during the investigations here described — mil be recognized by 
every member of our profession as one which it is exceedingly important 

* p. 157; conclusion 3; p. 158, line 4; p, 13'2, Nos. 21, 24. 



PLATE III 
^UTOaRA-PHIO STRAIN-DIAGRAMS OF METAT^ 



ILLUSTRATITO THE EFFECTS OF TFIWE jun ™ 




F.'6 . iS. 




f'6. 16 




3 



ng.i/. 




n^.is 




29 

to remove. It is hoped that it may not long remain, for it is evident 
that, although perhaps improbable, it is not imiDossible that, notwithstand- 
ing the increase of both tenacity and ductility by reduction of tempera- 
ture, these causes may still conspire to produce increased tendency to 
yield before shock at some unknown low temperature. 

36. The peculiarities of fracture, which have been alluded to, are ex- 
hibited in the accompanying Plate. 

Figures 15 and 16 exhibit the behavior of a bar of iron made at Cata- 
sauqua, Pennsylvania, as broken by Mr. Oliver Williams, at 70°, and, 
again, at 20^ Fahr. It is selected from among the specimens in the 
cabinets of the Stevens Institute of Technology. The fracture at 70° 
(Fig-ure 15) has the fibrous fracture and all the characteristics of what is 
generally considered a tough and ductile iron. That at 20° (Figure 16) 
resembles the break produced by a quick blow in good iron, or by any 
treatment at ordinary temperatures of a cold short iron. Were the 
conditions stated not known it would be supposed that irons like Nos. 1 
and 16, Figures 5 and 6, were combined in a single bar. 

Figures 17 and 18 illustrate strikingly the difference between the 
specimens of copper, Nos. 87 and 133, which have been ah-eady described 
and of which the strain-diagram of the one is shown in Plate II, and of 
the other in Plate HI. No. 87 was cast in green sand, and broken at 
70° F. , while the other was made from the same ingot, but cast in dry 
sand and broken at 10° Fahr. 

The first is unsound in structure in consequence of the dampness of 
the mould and exhibits a peculiar radiated texture which is probably due 
to the same cause. The second is distinguished by its compact, homoge- 
neous structure, probably due to the freedom of the mould from vapor 
and gases. It presents a beautiful crystalline fracture which is proba- 
bly due partially, if not principally, to the low temperature at which it 
was broken. 

These fractures are extremely interesting from the strongly tyjDical 
features which they exhibit as characteristic of the peculiar conditions 
under which they were produced. 

Resume. 

37. En resume, a review of this investigation of the nature of, and the 
influences affecting, the distortion and rupture of metals, it would seem 
allowable to accept, as extremely probable, the following 



30 

General Conltjsions : 

(1.) That accurate strain-diagrams, in which, the behavior of the 
material, as distortion progresses, and especially about the elastic limit, 
afford a means of acquiring valuable information respecting the strength, 
elasticity, homogeneousness, ductility and resilience of materials, and of 
tracing the modifications induced by variations of treatment and of com- 
position. 

(2.) That internal strain plays a most important part in determining 
the behavior of materials strained by either static or dynamic stress. 

(3. ) That the time, during which applied stress acts, is an important 
element in determining its effect, not only as an element which modifies 
the effect of the vis viva of the attacking force and the action of inertia of 
the piece attacked, but, also, as modifying seriously the conditions of 
production and relief of internal strain by even simple stresses. 

(4. ) That with good materials, cold does not produce injury but actu- 
ally improves their power of resisting stress and increases their resilience. 

(6. ) That the influence of impurities, of various methods of manufac- 
ture, of changes of density with temperature, and of the causes which 
produce a concentration of the action of rapidly produced distortion and 
of quick blows, are subjects which still require careful investigation. 

(7.) That experiment confirms the theory as to the behavior of mate- 
rials, homogeneous in composition, structure and strain, as expressed a 
priori in 27, and hence a probable deduction that the force of molecu- 
lar repulsion is heat motion. 



hS^^I^y of congress 



028 116 608 6 



py OF CONGRESS 

Hii 



